Field of the Invention
The present invention relates to an autofocus control technique in an image capturing apparatus.
Description of the Related Art
Conventionally known focus control methods used in digital cameras and other image capturing apparatuses include a phase-difference detection method and a contrast detection method (see Japanese Patent Laid-Open No. 09-054242 and Japanese Patent Laid-Open No. 2001-004914). Another known method is an imaging surface phase-difference detection method that takes into consideration a live-view (LV) mode in which image capture is performed while displaying captured images on, for example, a rear monitor (see Japanese Patent Laid-Open No. 2001-083407).
However, even with the imaging surface phase-difference detection method that takes the live-view mode into consideration, it is necessary to perform more stable focus control in conformity with the live-view mode and moving image capture. Especially with the recent trend toward higher resolution, a user easily feels a sense of discomfort if a focus state changes unexpectedly while moving images are displayed on a display unit.
In the imaging surface phase-difference detection method, the stability of focus control during the live-view mode and moving image capture may be improved by increasing a focus detection range. An increase in the focus detection range leads to an increase in the number of subjects that can be captured, and can alleviate an unexpected movement of a focus lens caused by movement of a temporarily-captured subject to the outside of the focus detection range.
However, when there is a conflict between a far point and a near point of a subject, an increase in the focus detection range could possibly make it difficult to bring the intended subject into focus. In view of this, it is considered that dividing the focus detection range into a plurality of focus detection areas is effective. However, in the case of the imaging surface phase-difference detection method, a subject image expands when a subject is significantly out of focus. This casts significant influence on the divided focus detection areas, thereby making it difficult to obtain focus detection results. Furthermore, during image capture with low light intensity, especially the signal-to-noise ratio is easily lowered in the divided focus detection areas, thereby making it difficult to obtain desired focus detection results. The same goes for the case of a low-contrast subject.
To address these problems, Japanese Patent Laid-Open No. 2014-32214 suggests the following technique: a phase-difference detection area is divided into a plurality of areas in a direction perpendicular to a direction of phase-difference detection, and in each of the divided areas, signals detected by pixels arranged along the direction of phase-difference detection are merged in the direction perpendicular to the direction of phase-difference detection. By thus merging the detected signals for reduction of the influence of noise, and by thus limiting a range of merger, an average pattern is yielded after merger of pixel signals, and a decrease in the precision of focus detection can be prevented. However, in Japanese Patent Laid-Open No. 2014-32214, the detected signals are merged only in the direction perpendicular to the direction of phase-difference detection, and there is a possibility that appropriate detected signals are not always obtained through merger depending on the image capturing condition.